The present embodiments relate to wireless communication systems and, more particularly, to operation of a Time Division Duplex communication system with dynamic reconfiguration of downlink (DL) and uplink (UL) time slots over which a user equipment (UE) communicates with one or more base stations.
With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are cell-specific reference signals (CRS), channel state information reference signals (CSI-RS), and demodulation reference signals (DMRS). These reference signals are useful for channel and interference measurement at the receiver. Cell-specific reference signals as well as channel state information reference signals are not precoded and are generated by a pseudo-random sequence generator as a function of the physical cell ID. In Releases 8 through 10 of the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS), which was designed for conventional point-to-point communication, the cell ID is not explicitly signaled by the base station (called eNB) but is implicitly derived by the UE as a function of the primary synchronization signal (PSS) and secondary synchronization signal (SSS). To connect to a wireless network, the UE performs a downlink cell search to synchronize to the best cell. A cell search is performed by detecting the PSS and SSS of each available cell and comparing their respective signal quality, for example, in terms of reference signal received power (RSRP). After the cell search is performed, the UE establishes connection with the best cell by deriving relevant system information for that cell. Similarly, for LTE Release 11 the UE performs an initial cell search to connect to the best cell. To enable multi-point CoMP operation, the connected cell then configures the UE by higher-layer signaling with a virtual cell ID for each CSI-RS resource associated with each respective base station involved in the multi-point CoMP operation. The UE generates the pseudo-random sequence for each CSI-RS resource as a function of the virtual cell ID.
Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is Release 8 of the 3GPP Long-Term Evolution. Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system. This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations.
FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102, and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102, and 103 (eNB) is operable over corresponding coverage areas 104, 105, and 106. Each base station's coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells. A handset or other user equipment (UE) 109 is shown in cell A 108. Cell A 108 is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 109. As UE 109 moves out of Cell A 108 into Cell B 107, UE 109 may be handed over to base station 102. Because UE 109 is synchronized with base station 101, UE 109 can employ non-synchronized random access for a handover to base station 102. UE 109 also employs non-synchronous random access to request allocation of uplink 111 time or frequency or code resources. If UE 109 has data ready for transmission, which may be user data, a measurements report, or a tracking area update, UE 109 can transmit a random access signal on uplink 111. The random access signal notifies base station 101 that UE 109 requires uplink resources to transmit the UE's data. Base station 101 responds by transmitting to UE 109 via downlink 110 a message containing the parameters of the resources allocated for the UE 109 uplink transmission along with possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on downlink 110 by base station 101, UE 109 optionally adjusts its transmit timing and transmits the data on uplink 111 employing the allotted resources during the prescribed time interval. Base station 101 configures UE 109 for periodic uplink sounding reference signal (SRS) transmission. Base station 101 estimates uplink channel quality indicator (CQI) from the SRS transmission.
Traditional wireless communication systems operate in either Frequency Division Duplex (FDD) or Time Division Duplex (TDD) modes. In a FDD mode, a pair of radio frequency (RF) carriers is assigned respectively to the downlink and uplink directions of the communication system. In contrast, a TDD system operates by time-multiplexing uplink and downlink transmissions within a fixed time interval on the same RF carrier. The ratio between UL and DL transmissions in the fixed time interval may be selected according to UL/DL data traffic patterns, or to support coexistence between dissimilar TDD wireless systems. The user equipment in a TDD system operates in a half-duplex mode, whereby it either receives from or transmits to the base station at any time instant but may not simultaneously transmit/receive.
FIG. 2, 200 is a diagram of a Long Term Evolution (LTE) TDD system. A radio frame of 10 milliseconds (ms) is partitioned into 1 ms subframes, where each subframe is either downlink (D), uplink (U) or a special subframe (S). There are seven Uplink-Downlink (UL/DL) configurations each with different uplink, downlink and special subframe patterns. For a cell under its control, an eNB selects one of the seven UL/DL configurations and broadcasts the configuration in system information. User equipment served by the eNB decodes the cell's system information to determine the correct uplink/downlink subframe configuration for the cell.
Referring now to FIG. 3, there is a diagram of a downlink subframe in LTE. Each subframe comprises twelve OFDM symbols with Extended Cyclic Prefix (CP) or fourteen OFDM symbols with Normal Cyclic Prefix (CP). The system bandwidth 315 consists of a plurality of L Physical Resource Blocks (PRB), where each PRB is composed of twelve OFDM tones called sub-carriers. The PRB is the smallest time-frequency resource allocation unit in LTE, where data transmission to a user is scheduled on one or multiple PRBs. Different PRBs in one subframe 301 are allocated for data transmission to different users. Furthermore, the set of PRBs on which a user receives downlink data transmission may change from one subframe to another.
Referring now to FIG. 4, there is a diagram of a special subframe in the LTE TDD system. The special subframe 400 consists of a downlink pilot time slot (DwPTS) 401, a guard period 402 and an uplink pilot time slot (UpPTS) 403. The guard period (GP) 402 enables the user equipment to switch from reception mode to transmission mode. The GP duration may also be dimensioned to support coexistence between different TDD systems such as coexistence between LTE TDD and Time-Division Synchronous Code Division Multiple Access (TD-SCDMA). Downlink data transmission may take place in the DwPTS region 401 which supports between three and twelve OFDM symbols. The UpPTS region 403 consists of one or two OFDM symbols and may be used to either transmit on the Physical Random Access Channel or to transmit SRS to the eNB.
In addition to downlink data, a base station also needs to transmit control information to mobile users. This includes both common control information as well as user-specific control information. Common control information is transmitted to all users in the cell to maintain users' connection to the network, page users in idle mode when a call comes in, schedule random access response, and indicate critical system information changes in the cell. In addition, user-specific control information is transmitted to each scheduled user, for example, to indicate the frequency resources on which the UE is expected to receive downlink data or transmit uplink data. Referring back to FIG. 3, each LTE subframe is divided into legacy control region 306 for downlink control information transmission and data region 307 for downlink data transmissions. The legacy control region 306 comprises OFDM symbols 1-3 when the system bandwidth is greater than 10 PRBs and OFDM symbols 2-4 otherwise. The exact size of the legacy control region is signaled on a Physical Downlink Control Format Indicator Channel (PCFICH). The data channel region 307 is located after the legacy control channel region 306 and is allotted for each Physical Resource Block (PRB). The legacy control channel region 306 is a region to which a Physical Downlink Control Channel (PDCCH) is mapped. The data channel region 307 is a region to which a Physical Downlink Shared Channel (PDSCH) is mapped and carries downlink data transmission to mobile users. Further, Enhanced Physical Downlink Control Channels EPDCCH Set 1 309 and EPDCCH Set 2 313 are frequency multiplexed with the data channel (PDSCH) 311 for transmission to a UE. That is, EPDCCH Set 1 309 and EPDCCH Set 2 313 are mapped to the data channel region 307 together with the PDSCH 311. The reason to locate the legacy control channel region at the beginning of the subframe is that a UE firstly receives a PDCCH allotted to the legacy control channel region 306 to recognize the presence of transmission of the PDSCH. Once the presence of transmission of the PDSCH is recognized, the UE may determine whether to perform a receiving operation of the PDSCH. If no PDCCH is transmitted to the UE, it is unnecessary to receive the PDSCH mapped to the data channel region 307. Accordingly, the UE may save power consumed in a receiving operation of the PDSCH. Meanwhile, the UE may receive a PDCCH located in the control channel region faster than the PDSCH 311 to reduce a scheduling delay. However, because the PDCCH is transmitted over the entire system bandwidth, interference control is impossible.
The legacy control channel region 306 may not be changed to a frequency multiplexing structure to maintain compatibility with an existing or legacy UE. However, if the eNodeB does not allot a corresponding region of the data channel region 307 to a UE of a previous LTE version, the UE of a previous LTE version does not receive a resource mapped to a corresponding data channel region 307. Accordingly, the eNodeB may transmit an EPDCCH for a UE of a new LTE version in a data channel region 307 that is not allotted to the UE. In other words, an EPDCCH being a control channel for a UE of a new LTE version has a structure multiplexed with the PDSCH.
FIG. 5 is a diagram of a Physical Resource Block (PRB) pair. The eNB may configure 1, 2, 4, or 8 PRB pairs for transmission to the UE. However, each PRB pair is a replica, and only one PRB pair is shown for the purpose of explanation. Each column of the diagram of the subframe corresponds to 12 subcarriers or tones in an OFDM symbol. There are 14 OFDM symbols in the subframe with a normal cyclic prefix (CP). The 3 OFDM symbols on the left side of the subframe include resource elements (REs) for transmission of a legacy physical downlink control channel (PDCCH) and legacy cell-specific reference signals (CRS). These 3 OFDM symbols are necessary for backwards compatibility with previous wireless standards. The 11 OFDM symbols on the right include resource elements (REs) for transmission of an enhanced physical downlink control channel (EPDCCH), and demodulation reference signals (DMRS), as well as cell-specific reference signals (CRS) and orphan or unused REs. Orphan REs may exist because the UE shall always assume that 24 REs are reserved for DMRS transmission in a PRB pair configured for EPDCCH transmission.
While the preceding approaches provide steady improvements in interference measurement and Channel State Information reporting for wireless communications, the present inventors recognize that still further improvements are possible. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art.